Variability in mussel settlement on suspended ropes placed at Ahipara Bay, Northland, New Zealand

Unique physical and biological conditions at Ninety Mile Beach, northern New Zealand, are responsible for an accumulation of unusually high amounts of mussel spat, Perna canaliculus, found nowhere else in the country. These massive quantities of mussel spat are collected from the beach (>140,000 tonnes/year) and transported to the aquaculture farms, where they supply 80% of the industry's mussel seed requirements. Direct settlement of mussels, P. canaliculus, from the plankton to suspended ropes in the water column was investigated at Ninety Mile Beach, northern New Zealand. Mussel spat-collecting ropes were placed at two sites (inside and outside Ahipara Bay) and at three different depths. The abundances of mussels within three size classes were recorded monthly during two spawning seasons in 1999 and 2000. Small mussels (<0.49 mm) were found to be more abundant at shallower depths (2 m water depth) in August 2000, just after the spawning of nearby mussel populations. Conversely, larger mussels (>1.0 mm) were relatively more abundant at greater depths (18 m water depth) in September-December. Larger mussels are thought to have transferred from tumbling algae covered with mussels, which are readily transported by hydrodynamic processes near the seafloor. Although higher mussel abundances generally were found at the site inside the bay, these differences were not strongly supported by statistical analyses. The present research provides the first evidence that artificial ropes deployed at Ahipara Bay may be of commercial value to spat collectors on the beach and the mussel farm industry. These ropes could be used as predictors of spatfall arrivals, which are collected at the beach and used to seed the mussel farms. Furthermore, ropes could possibly be used as a new and more reliable means of harvesting the massive quantities of mussel spat available at Ninety Mile Beach, northern New Zealand.